Media charges that Tony Blair was "Napoleonic" in style were always a bit rich. Ever tried contradicting a tabloid newspaper editor - or Conrad Black, the Daily Telegraph's Bonaparte-fancying proprietor? But the dirigiste nature of the Blair regime was established early on. His is not a collegial style. Cabinet meetings are brief and none too frequent. There were some 87 a year under Clement Attlee when Labour was in power in the 40s, 60 under Ted Heath in the 70s but only 40 under John Major in the 90s. Mr Blair, who calls even fewer, follows a trend in his preference for dealing with colleagues bilaterally rather than in conclave.

In a new dissection of Downing Street*, Peter Hennessy is on the look-out for "worrying features of excessive prime ministerialism" during the Blair years. It is not just because over-centralisation of decision-making could lead to abuse of power. Prime ministers who try to do too much burn themselves out. You need - the dictum is attributed to the great nearly man Roy Jenkins - at least four or five years to have a serious chance of shifting the British polity, yet such is the punishing nature of the job that exhaustion and negativism set in all too early.

But the idea that the Blair years have seen irrevocable or harmful shifts in the way we are governed is quickly scotched - devolution aside. Professor Hennessy, who teaches at Queen Mary and Westfield College in London, recalls the near panic in 1974 when Tory prime minister Ted Heath clung to office despite having lost the election. Senior civil servants paced anxiously round St James's Park - where were the precedents, who was the arbiter of right conduct?

There is not much of a template for the powers of a PM. No law sets rules or requirements (though Tony Benn did propose such a thing). The rough-and-ready code that governs the conduct of ministers applies to the PM, but there is no one to adjudicate it - except perhaps Whitehall's head prefect, the cabinet secretary. To his credit, the incumbent, Sir Richard Wilson, has avoided talk about the primordial principles of British constitutionalism. As far as the premiership goes, there are none.

As Herbert Asquith, the Liberal prime minister, said - in between writing letters to his mistress during cabinet sessions - the job is pretty much what any given prime minister makes of it. Tony Blair's tenure comes from his personal dominance of his party and from his Commons majority. It is also hugely constrained by the power of his chancellor, Gordon Brown, a relationship for which there are few precedents (though Harold Macmillan's with Anthony Eden in the mid-50s might compare). None of Napoleon's generals got anywhere close to the "bi-stellar" relationship between No 10 and No 11.

Pursuing the analogy, we have to note too that Bonaparte never gave Corsica away: devolution of power to Scotland and Wales makes the Blair administration "unarguably different" from its predecessors. The scale of recent constitutional innovations, human rights and all, has been camouflaged by the prime minister's aversion to talking about it in the round.

All recent prime ministers have dignitaries to see and appointments to make. The chart shows the changing pattern of business from the 40s to the 60s, which is as far as we can go thanks to the 30-year rule for release of cabinet papers. We can surmise that Mr Blair spends time on presentation, meeting columnists and entertaining the editor of the Daily Mail - a singularly fruitless task by the look of it. Professor Hennessy observes dryly that his hero Clem Attlee "would not only have found the media life of Mr Blair incredible, he would have recoiled from it absolutely".

Yet he finds Mr Blair in tune with his predecessors in one critical respect - how to wage war. Mr Blair is the first post-1945 premier to have presided over two conflicts in the space of six months if we exclude colonial emergencies. His mettle was tested and he proved remarkably collegial in the crises. The full cabinet was consulted over Kosovo prior to Nato's air strikes and kept regularly updated after that. One member quipped there was more collective discussion of the Balkans than the Budget. Then and during the Gulf emergencies, a version of the traditional war cabinet convened, attended by foreign secretary Robin Cook, defence secretary George Robertson, the attorney general and Alastair Campbell - and there are precedents for personal advisers as close-in as he has been. In the ultra-private business of government, for example consuming the products of the intelligence agencies, Mr Blair has broken no moulds.

A perennial problem in the governance of the UK is determining how much power and influence the centre should have over departments, to keep them in line without stifling initiative. Despite civil service reforms, despite Mr Blair's personal ascendancy, it is far from obvious that the New Labour solution is any more satisfactory or long-lasting than those sought by Harold Wilson or Margaret Thatcher, two other tinkerers with the machinery of the central state.

The window for structural change at the centre is now shutting. As the general election approaches and events take their toll, it is a fair bet that cabinet discussions will lengthen. Consensus will again be prized. Nothing of the usual way of doing prime ministerial business has been thrown away since May 1997.

*Peter Hennessy's The Prime Minister: The Office and its Holders Since 1945 is published by Allen Lane.